This invention relates to a fluid-bed catalytic process for upgrading olefinic light gas feedstock (termed "light gas" for brevity herein) containing lower, particularly C.sub.3 -C.sub.5, olefins (alkenes) and paraffins (alkanes). The olefins are converted to distillate in a single-zone fluid-bed reactor, operating at moderate pressure and temperature, ("low severity" conditions). These operating conditions are referred to herein as "easy", and the process is referred to as a low severity MOD (for "Mobil Olefin to Distillate") process. By "distillate" we refer to C.sub.9.sup.+ hydrocarbons boiling in the range from about 130.degree. C. to about 300.degree. C. (266.degree. F. -572.degree. F.). We know of no other process which can convert olefins with at least 80% conversion and about 20% by weight (wt %) single pass yield, to distillate, in a single zone.
More particularly, the invention provides a continuous process for oligomerizing light gas containing a C.sub.3 -C.sub.5 olefin, namely propene, butenes and pentenes, a mixture thereof, or mixtures thereof with a minor proportion by weight of paraffins, preferably only C.sub.1 -C.sub.5 paraffins, in the absence of added hydrogen, to a C.sub.9.sup.+ rich hydrocarbon stream, in a highly efficient operating mode because of a fluid catalyst bed with "tailored" activity. By "C.sub.9.sup.+ rich" we refer to the presence of at least 10% by weight of C.sub.9.sup.+ hydrocarbons in the product. By "tailored" activity we refer to a bed which consists essentially of weight fractions, in a specified range, of equilibrated low activity catalyst particles, the weight fraction in each range having a specified activity (alpha) in defined ranges below 10, and less than 10 wt % of the catalyst has an activity greater than 10.
The alpha value for catalyst activity is defined by the specific test described in U.S. Pat. Nos. 3,827,968 and 3,960,978 to Givens et al, the disclosures of which are incorporated by reference thereto as if fully set forth herein.
Developments in fluid-bed catalytic processes using a wide variety of zeolite catalysts have spurred interest in commercializing the conversion of olefinic feedstocks for producing C.sub.5.sup.+ gasoline, diesel fuel, etc. In addition to the discovery that the intrinsic oligomerization reactions are promoted by ZSM-5 type zeolite catalysts, several discoveries relating to implementing the reactions in an apt reactor environment, have contributed to the commercial success of current industrial processes. These are environmentally acceptable processes for utilizing feedstocks that contain lower olefins, especially C.sub.3 -C.sub.5 alkenes, though some ethene (ethylene), and some olefins and paraffins heavier than C.sub.5 may also be present.
Of particular interest is that the ZSM-5 type catalyst used under our "easy" process conditions, and also as used under the severe process conditions of Ser. No. 006,407, does not appear to suffer from a sensitivity (poisoning) to basic nitrogen-containing organic compounds such as alkylamines (e.g. diethylamine), or, to oxygenated compounds such as ketones, a proclivity which is characteristic of the catalyst under the process conditions of prior art olefin oligomerization processes, particularly the fixed bed processes operated at very high pressure. Such processes require the addition of hydrogen as a preventitive antidote. It will be recognized that alkylamines are used in treating light gas streams, and ketones are typically present in Fischer Tropsch-derived light ends streams, both of which streams are particularly well-suited for upgrading by oligomerization. Though our process is not adversely affected by the presence of hydrogen, there is no readily discernible economic incentive for using it in our single stage reactor, and we prefer not to do so.
Prior art moderate pressure processes using a zeolite catalyst to oligomerize lower olefins under comparable temperature conditions produced excellent conversions to distillate range olefins in a fixed bed microreactor but neglected to state what the alpha value of their catalyst was; nor did they suggest that the alpha value might be of over-riding significance (see "Conversion of C.sub.2 -C.sub.10 Olefins to Higher Olefins Over Synthetic Zeolite ZSM-5" by W. E. Garwood presented at the Symposium on Advances in Zeolite Chemistry before the Division of Petroleum Chemistry, Inc., American Chemical Society, Las Vegas Meeting Mar. 28-Apr. 2, 1962).
When the low severity process is carried out in a fluid bed using a relatively "high alpha" catalyst (above 100), the result is formation of a mixture of aromatics, naphthenes and paraffins, and a minor proportion by wt of olefins. Even when our process is carried out in a fluid bed with a relatively uniform low alpha in the range from 1 to 5, that is, none of the catalyst has an alpha greater than 5, niether the high conversion to C.sub.5.sup.+ olefins, nor the high selectivity to C.sub.9.sup.+ is obtained.
Conversion of C.sub.3 -C.sub.5 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 3,760,024) and Yan et al (U.S. Pat. No. 3,845,150) to be effective processes using the ZSM-5 type zeolite catalysts. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity; so do Dwyer et al in U.S. Pat. No. 3,700,724. Garwood et al have also contributed to the understanding of catalytic olefin upgrading techniques and have contributed improved processes as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The '062 patent discloses conversion of olefins to gasoline or distillate in the range from 190.degree.-315.degree. C. and 42-70 atm; and this, and the '640 and '992 disclosures are incorporated by reference thereto as if fully set forth herein.
The '978 patent discloses that low alpha ZSM-5 and ZSM-11 catalysts not only have reduced activity for cracking n-hexane and other paraffins, but also produce less than 10% by wt aromatics. The runs were made in a fixed bed microreactor, and, at that time, it was not known that the process was not operable on a larger scale without the addition of hydrogen to control coke deposition and to prevent poisoning of the catalyst by nitrogen-containing organic impurities. The basic knowledge that low activity ZSM-5 and ZSM-11 type catalysts effectively oligomerized lower olefins was used to arrive at improvements in "Conversion of LPG Hydrocarbons to Distillate Fuels or Lubes Using Integration of LPG Dehydrogenation and MOGDL" in U.S. Pat. No. 4,542,247 to Chang et al which discloses fixed beds in a two-stage catalytic process for converting olefins to gasoline and distillate; and, more recently, in "Catalytic Conversion of Olefins to Higher Hydrocarbons" in U.S. Pat. No. 4,456,779 to Owen et al. which discloses oligomerization of olefins in three down-flow fixed beds, in series, with intercoolers. Both fixed-bed processes require the addition of hydrogen for the reasons given hereinabove.
The low severity conditions under which our process produces distillate from lower olefins is particularly unexpected because it was generally accepted that relatively high severity was essential to obtain conversions of economic significance. Never has there been any suggestion that a tailored weight fraction distribution of equilibrated low activity catalyst particles might have a dominating effect on the oligomerization of a lower olefin feed if the process conditions were aptly chosen.
The prior art processes relate to the conversion of lower olefins, especially propene and butenes, over ZSM-5 and HZSM-5, at moderately elevated temperatures and pressures. The sought-after conversion products are liquid fuels, especially the C.sub.6.sup.+ aliphatic and aromatic hydrocarbons. It is known that the product distribution may be tailored by controlling process conditions, such as temperature, pressure and space velocity. Gasoline (C.sub.6 -C.sub.10) is readily formed at elevated temperature (preferably about 400.degree. C.) and pressure from ambient to about 2900 kPa (420 psia), preferably about 250 to 1450 kPa (36 to 210 psia). Olefinic gasoline can be produced in good yield and may be recovered as a product or fed to a low severity, high pressure reactor system for further conversion to heavier distillate-range products. Distillate mode operation can be employed to maximize production of C.sub.9.sup.+ aliphatics by reacting the lower and intermediate olefins at high pressure and moderate temperature. Operating details for typical "MOGD" (for Mobil Olefin to Gasoline & Distillate) oligomerization units are disclosed in U.S. Pat. Nos. 4,456,779 and 4,497,968 (Owen et al); 4,433,185 (Tabak); and U.S. Patent application Ser. No. 006,407.
We have now found that C.sub.3 -C.sub.4 -rich and higher olefins may be selectively upgraded to normally liquid hydrocarbons in the distillate range by catalytic conversion in a turbulent fluidized bed of solid acid ZSM-5 type of zeolite catalyst at least 90 wt % of which has an equilibrated alpha less than 10, and the remainder an alpha above 10, most preferably operating at a pressure below 1480 kPa (200 psig) in the dense phase, in the absence of added hydrogen, in a single pass, or with recycle of undesired oligomerized product.
Such operation results in the most important advantage of our MOD process, namely the use of a relatively low cost, low pressure reactor in which close temperature control is afforded by operation of a fluid-bed in the turbulent regime (referred to as a "turbulent bed"). An essentially uniform conversion temperature may be maintained (often with closer than +5.degree. C. tolerance) to ensure that the distribution of activity of the catalyst weight fractions lies generally along a predetermined curve. Except for a small zone adjacent the bottom gas inlet, the midpoint measurement of conditions in the bed is representative of the entire bed, due to the thorough mixing achieved.
Moreover, in turbulent beds, fluidization is better at a higher fluidizing gas velocity, and with a higher level of the finer sizes of catalyst (see R. M. Braca and A. A. Fried, in Fluidization, D. F. Othmer, Ed. (Reinhold, New York, 1956), pp. 117-138; W. W. Kraft, W. Ulrich, W. O'Connor, ibid., pp. 184-211). This requires a significant amount of fines, from about 10 to 25% by weight (% by wt) having a particle size less than 32 microns. Since it is difficult to control the distribution and activity of catalyst fines it would seem at cross-purposes deliberately to require operation with a predetermined weight fraction of fines.
U.S. Pat. Nos. 4,417,086 and 4,417,087 to Miller teach a two-zone reactor operating in the transport mode where the relative superficial gas velocity is greater than the terminal velocity in free fall. There is no suggestion that the activity of the bed be tailored by specifying weight fractions of catalyst particles, all having relatively low equilibrated activity. Though the operation of a fluid-bed is illustrated (example 2 in each of the '086 and '087 patents) note that no operating pressure is stated in the former, and that operating pressure in the latter is 10 psig (24.7 psia, 170 kPa). The general disclosure that the processes may be operated at a pressure in the range from subatmospheric to several hundred atmospheres, but preferably 10 bar or less, and most preferably 0 to 6 bar, (see middle of col 6 in 086, and, near top of col 5 in '087) is not so ingenuous as to be meant to apply equally to the fixed bed (example 1 of '086 and '087, each illustrates 34.5 bar, 500 psi) and the 170 kPa fluidbed.